This invention relates to a heat pump fluid heating system for producing hot fluid at temperatures at least equal to the condensing temperature in a heat pump system. In particular, the present invention relates to a heat pump fluid heating system for producing hot water at high temperatures, suitable for use as a processing heat source such as in a milk pasteurizing system.
Heat pump fluid heating systems are used for example to heat water for various applications such as for domestic hot water, or swimming pools.
These systems generally utilize a heat pump cycle using a compressor, a condenser, and evaporator. In the case of domestic water heating where higher temperatures are required, the water may be heated to a high temperature using the superheat from the superheated working fluid exiting the compressor.
U.S. Pat. No. 5,901,563 to Yarbrough et. al. discloses a heat pump heat transfer system which includes a refrigerant to water heat exchanger, known in the art as a desuperheater, for transferring superheat from the compressed gas exiting the compressor to a domestic hot water service. This enables higher temperatures to be reached as required for domestic hot water systems. However, water is only heated at the desuperheater, and while a high temperature can be obtained, the flow rate is small.
For other applications such as for a processing heat source however, heat pumps have had little application, due to their inability to produce useful flowrates at the required higher temperatures, stemming from the fact that the flow of fluid to be heated (referred to hereunder as heated fluid) necessary for the working fluid condensation is considerably greater than is required to de-superheat the same working fluid, yet only the latter phase possesses the capacity to raise the heated fluid to higher temperatures. This imbalance results in either the provision of a full heated fluid flow at generally lower temperatures, or as with Yarbrough, a small flow at a higher temperature. In this case, the lower temperature balance is of little or no value, unless low temperature applications are available.
FIG. 1 shows a conventional heat exchanger configuration for hot gas cooling of a heat pump system. With this configuration, a heat exchanger 1 is configured with a working fluid inlet 2 and outlet 3, and a coolant (heated fluid) inlet 4 and outlet 5. This configuration provides a reasonable output flowrate, but only at medium temperatures, being unsuited to most requirements for high temperature heated water.
The problem of obtaining higher flow rates for a high temperature system is somewhat overcome by U.S. Pat. No. 4,474,018 to Teagan which discloses a heat pump system for production of domestic hot water, which involves using a compressor section which provides working fluid in a multiplicity of pressures. With this arrangement, water is heated in series connected heat exchangers, each provided with condensing coils in separate loops. Having the condensing coils in separate loops enables the plant to be designed for optimum performance, since flow rates and temperatures can be varied for the separate loops. With this design each of the heat exchanger/condensor sections combine desuperheating and condensing, and are in effect the same as shown in FIG. 1. While having separate loops enables design for optimum performance, this adds to the complexity of the system and hence cost and size.
Furthermore, neither of the above patents disclose the use of a liquid/gas heat exchanger to improve the system economy by transferring heat between the working fluid output from the condenser and the working fluid input to the compressor. Nor do they disclose the possibility of also using the heat pump to concurrently provide chilled water, such as is required for example in a milk pasteurizing plant.
It is an object of the present invention to address the above problems, and provide a heat pump fluid heating system which enables a compact design, and which can achieve sufficient flows of high temperature fluid for use in processing plants such as for sterilizing, and pasteurizing.
Moreover it is an object to provide a method of determining the required heated fluid mass flow rate and heated fluid entering temperature for such a heat pump fluid heating system.
According to one aspect of the present invention there is provided a heat pump system for raising the temperature of a heated fluid, comprising;
a compressor for compressing a working fluid,
a desuperheater heat exchanger provided with an inlet and outlet for the heated fluid and an inlet and outlet for the working fluid, the working fluid inlet being communicated with an outlet from the compressor,
a condenser heat exchanger provided with an inlet and outlet for the heated fluid and an inlet and outlet for the working fluid, the condenser heat exchanger heated fluid outlet being communicated directly with the desuperheater heat exchanger heated fluid inlet, and the condenser heat exchanger working fluid inlet being communicated directly with the desuperheater heat exchanger working fluid outlet, and
an evaporator with an inlet communicated with the condenser heat exchanger working fluid outlet, and an outlet communicated with an inlet to the compressor.
The compressor may be any suitable device such as a rotary compressor, a screw compressor or a reciprocating compressor, in either single or multiple stages. Moreover, two or more compressors may be provided as required.
The evaporator may be any conventional evaporator used for a heat pump system, such as an air cooled or liquid cooled evaporator. In the case where process cooling is also required, the evaporator may be a liquid cooled heat exchanger adapted for connection to a liquid recirculation system, for providing cooling.
The desuperheater heat exchanger and the condenser heat exchanger may be arranged in any suitable configuration, provided these are connected in series. For example the desuperheater heat exchanger may be arranged above the condenser heat exchanger so that any condensate from the desuperheater heat exchanger will flow down into the condenser heat exchanger.
In a preferred embodiment, where economy of space is a prerequisite, the desuperheater heat exchanger may be arranged so that a working fluid outlet therefrom is below an inlet to the condenser heat exchanger, and there is provided a device for carrying any condensate into the condenser heat exchanger inlet.
With this arrangement, the desuperheater heat exchanger and the condenser heat exchanger may be arranged side by side, thus providing a compact arrangement.
The device for carrying condensate may comprise any suitable device. For example this may comprise piping between the heat exchangers sized and formed so that any condensate from the desuperheater heat exchanger is carried by flow of gaseous working fluid into the inlet of the condenser heat exchanger. A typical arrangement man involve a standard xe2x80x9cPxe2x80x9d trap.
According to another aspect of the present invention the heat pump system as described above is further provided with a liquid/gas heat exchanger arranged and configured so as to transfer heat from the working fluid output from the condenser heat exchanger to the working fluid input to the compressor.
The invention also covers a method of determining heated fluid mass flow rate and heated fluid entering temperature for a heat pump system comprising a desuperheater heat exchanger and a condensor heat exchanger connected in series with a heated fluid flowing in series through the desuperheater heat exchanger and condensor heat exchanger, comprising the steps of;
specifying a required heated fluid discharge temperature A, a required working fluid condensing temperature B, a required desuperheater heat exchanger duty C, a required condenser heat exchanger duty D, a temperature difference between the working fluid and heated fluid at exit of the condenser heat exchanger F, and the specific heat capacity of the heated fluid G;
determining a heated fluid mass flow rate H according to the following formula;   H  =      C          G      ⁡              [                  A          -                      (                          B              -              F                        )                          ]            
and then determining a heated fluid entering temperature E according to the following formula;   E  =            (              B        -        F            )        -          (              D                  G          xc3x97          H                    )      
The invention also covers a heat pump system for raising the temperature of a fluid, comprising a desuperheater heat exchanger and a condenser heat exchanger connected in series, wherein required heat transfer duties of the desuperheater heat exchanger and the condenser heat exchanger are determined so that a fluid passed in series through these heat exchangers when operating at specified condensing and evaporating temperatures of a working fluid, becomes heated to a specified temperature of at least the condensing temperature of the working fluid.